Golf club steel shaft

ABSTRACT

A golf club steel shaft in which, when a distance from a front end of the shaft to a center of gravity point of the shaft is L G  and when a full length of the shaft is L S , a center-of-gravity rate Y (%) obtained as L G /L S  satisfies 51.0≦Y≦60.0, a shaft weight X (g) satisfies 80≦X≦140, and a full length of the shaft L S  (inch) satisfies 30≦L S ≦42.

TECHNICAL FIELD

The present invention relates to a steel shaft.

BACKGROUND ART

For golfers, flight distance of a ball is one of the important factorswhen selecting a golf club. Therefore, hitherto, in order to extend theflight distance of the ball, various improvements have been made withregard to shapes and materials of elements forming a golf club.

However, in recent years, in order to enhance fairness of competition bysuppressing excessive flight distance, there have been regulations setin the rules regarding rebound performance of a clubface, club length,and inertia moment of a head; and thereby it is becoming difficult toimprove flight distance.

In such a situation, in view of the fact that initial velocity of a balllargely influences flight distance, there has been a proposal (forexample, cf. Patent Literature 1) of extending the club length close tothe upper limit regulated by the rule to increase head speed of a club.

CITATION LIST Patent Literature

-   [PTL1] Japanese Laid-Open Patent Publication No. 2004-201911

SUMMARY OF THE INVENTION Technical Problem

However, with the method of increasing head speed of a club by extendingthe club length, controllability of the head deteriorates as the lengthof the club becomes longer, and it becomes difficult to hit a ball at asweet spot of the head. Thus, a ball smash factor deteriorates andinitial velocity of a ball cannot be stably increased; and, as a result,flight distance of a ball cannot be improved.

In order to solve this, it is necessary to increase the smash factor byreducing the length of the club and increase initial velocity of theball by increasing the head weight. However, simply increasing the headweight leads to a problem where ease of swinging the club decreases dueto inertia moment of the club now becoming large.

Therefore, it is conceivable to move the center of gravity of the shafttoward the butt side (hand side) in order to prevent the increase ofinertia moment of the club without further increasing the club weight.

Although it is ordinarily possible to move the center of gravity of theshaft toward the butt side by increasing the thickness of the a buttside portion of the shaft; with this method, a flexural rigidity valueEI (kgf·m²) of the butt side portion of the shaft also increases, andfeel of hitting a ball and directivity of a hit ball deteriorate.

The present invention has been made in view of such circumstances, andan objective of the present invention is to provide a golf club steelshaft that makes it possible to extend flight distance of a ball whilemaintaining feel of hitting a ball and directivity of a hit ball.

Solution to the Problems

(1) In a golf club steel shaft of the present invention,

when a distance from a front end of the shaft to a center of gravitypoint of the shaft is L_(G) and when a full length of the shaft isL_(S), a center-of-gravity rate Y (%) obtained as L_(G)/L_(S) satisfies51.0≦Y≦60.0,

a shaft weight X (g) satisfies 80≦X≦140, and

a full length of the shaft L_(S) (inch) satisfies 30≦L_(S)≦42.

In the golf club steel shaft of the present invention, since thecenter-of-gravity rate Y (%), the shaft weight X (g), and the fulllength of the shaft L_(S) (inch) are each set as values within the abovedescribed ranges, it is possible to increase weight of a golf club headwith steel iron, and extend flight distance of a ball while maintainingfeel of hitting a ball and directivity of a hit ball.

(2) In the golf club steel shaft according to (1) described above, athickness of a portion up to 150 mm from a butt end toward a front endside of the shaft is preferably not smaller than 0.30 mm but not largerthan 0.40 mm.

(3) In the golf club steel shaft according to (1) or (2) describedabove, an outer diameter of a portion up to 150 mm from a butt endtoward a front end side of the shaft is preferably not smaller than 14.5mm but not larger than 15.3 mm.

(4) In the golf club steel shaft according to (1) to (3) describedabove, the number of steps included in a portion up to 400 mm from abutt end toward a front end side of the shaft is preferably not lessthan 5 but not more than 8.

Advantageous Effects of the Invention

With the steel shaft according to the present invention, it is possibleto extend flight distance of a ball while maintaining feel of hitting aball and directivity of a hit ball.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an illustrative diagram of a golf club including oneembodiment of a golf club steel shaft of the present invention.

FIG. 2 is for describing a method for measuring T-point strength.

DESCRIPTION OF EMBODIMENTS

In the following, detailed embodiments of a steel shaft for a golf clubof the present invention will be described with reference to theaccompanying drawings.

FIG. 1 is an illustrative diagram showing the entirety of a golf club 1including a steel shaft (hereinafter, may be simply referred to as“shaft”) for a golf club, according to one embodiment of the presentinvention. The golf club 1 includes an iron type golf club head 2 havinga predetermined loft angle, a shaft 3, and a grip 4. The head 2 has ahosel 6 including a shaft hole 5 to which a tip end 3 a on the front endside of the shaft 3 is inserted and fixed. A butt end 3 b at the backend side of the shaft 3 is inserted and fixed in a grip hole 7 of thegrip 4. The tip end 3 a is positioned inside the head 2, and the buttend 3 b is positioned inside the grip 4. It should be noted that, inFIG. 1, a reference character of “G” indicates the center of gravity(center of gravity point) of the shaft 3. The center of gravity G islocated on a shaft axis inside the shaft 3.

Although the weight of the golf club 1 is not particularly limited inthe present invention, it is preferably set within a range of notsmaller than 360 g. If the weight of the golf club 1 is too small, thestrengths of respective elements (parts) forming the golf club 1 becomelow, and durability of the golf club 1 may deteriorate. Therefore, theweight of the golf club 1 is preferably not smaller than 370 g, andfurther preferably not smaller than 380 g. On the other hand, if theweight of the golf club 1 is too large, it becomes difficult to performa swing, and it becomes difficult to increase head speed. Therefore, theweight of the golf club 1 is further preferably not larger than 500 g,and particularly preferably not larger than 490 g.

Furthermore, the length of the golf club 1 itself is also notparticularly limited in the present invention, and is ordinarily from34.0 to 42.0 inches. If the length of the golf club 1 is too small,although a swing can be performed easily, a turning radius of the swingbecomes small, and it becomes difficult to obtain a sufficient headspeed. As a result, the ball speed cannot be increased, and the flightdistance of the ball cannot be extended. Therefore, the length of thegolf club 1 is preferably not smaller than 34.5 inches, and furtherpreferably not smaller than 35.0 inches. On the other hand, if thelength of the golf club 1 is too large, the head speed decreases sinceit becomes difficult to swing the club. Therefore, the ball speed cannotbe increased, and the flight distance of the ball cannot be extended.Therefore, the length of the golf club 1 is preferably not larger than41.5 inches, and further preferably not larger than 41.0 inches.

It should be noted that, in the present specification, “club length” isa length measured based on the description in “Appendix II—Design ofClubs” “1. Clubs” “1c. Length” in the Rules of Golf determined by R&A(The Royal and Ancient Golf Club of Saint Andrews).

[Head Configuration]

The head 2 in the present embodiment is a solid iron head and has alarge inertia moment. With a club having the head 2 with a large inertiamoment, an advantageous effect of improvement in flight distance can bestably obtained.

In the present invention, there is no particular limitation in thematerial of the head 2, and, for example, titanium, titanium alloys,stainless steel, maraging steel, soft iron, and the like can be used.Furthermore, instead of manufacturing the head 2 using a singlematerial, the head 2 may be manufactured by combining multiple materialsas appropriate.

In the present invention, although the weight of the head 2 itself isnot particularly limited, it is preferably within a range from 230 to310 g. If the head 2 is too light, the kinetic energy of the head 2cannot be sufficiently provided to the ball, and it becomes difficult toincrease the ball speed. Therefore, the weight of the head 2 is furtherpreferably not smaller than 235 g, and particularly preferably notsmaller than 240 g. On the other hand, if the weight of the head 2 istoo large, the golf club 1 becomes heavy and difficult to swing.Therefore, the weight of the head 2 is further preferably not largerthan 305 g, and particularly preferably not larger than 300 g.

Furthermore, in the golf club 1 of the present embodiment, the ratio(head weight/club weight) of the head weight and the club weight is setto be not lower than 0.55 but not higher than 0.70. If this ratio is toosmall, the kinetic energy of the head 2 becomes small and obtaining asufficient ball speed becomes difficult. Therefore, the ratio ispreferably not lower than 0.56, and further preferably not lower than0.57. On the other hand, if the ratio is too large, the head 2 becomesheavy and it becomes difficult to swing the club. Therefore, the ratiois preferably not higher than 0.69, and further preferably not higherthan 0.68.

[Grip Configuration]

In the present invention, there is no particular limitation in thematerial and structure of the grip 4, and those commonly used can beadopted as appropriate. For example, there can be used one that isobtained by blending and kneading natural rubber, oil, carbon black,sulfur, and zinc oxide, and molding and vulcanizing the materials into apredetermined shape.

In the present invention, although the weight of the grip 4 itself isnot particularly limited, it can be ordinarily set to be not smallerthan 20 g but not larger than 55 g. If the weight of the grip 4 is toosmall, the strength of the grip 4 becomes low, and its durability maydeteriorate. Therefore, the weight of the grip 4 is preferably notsmaller than 23 g, and further preferably not smaller than 26 g. On theother hand, if the weight of the grip 4 is too large, the golf club 1becomes heavy and difficult to swing. Therefore, the weight of the grip4 is preferably not larger than 53 g, and further preferably not largerthan 51 g.

[Shaft Configuration]

The shaft 3 in the present embodiment is a steel shaft having a hollowstructure and is manufactured with a method commonly used in the art.The full length of the shaft 3 is represented as L_(S), and the distancefrom the tip end (front end) 3 a of the shaft 3 to the center of gravityG of the shaft 3 is represented as L_(G).

A weight X of the shaft 3 in the present invention is set to be notsmaller than 80 g but not larger than 140 g. If the weight of the shaft3 is too small, the possibility becomes high for strengths such asflexural strength to be insufficient due to having a small thickness.Therefore, the weight of the shaft 3 is preferably not smaller than 85g, and further preferably not smaller than 90 g. On the other hand, ifthe weight of the shaft 3 is larger than 140 g, it becomes difficult toperform a swing at an increased speed due to the whole golf club 1 beingheavy. Therefore, the weight of the shaft 3 is preferably not largerthan 135 g, and further preferably not larger than 130 g.

In addition, the length (full length) L_(S) of the shaft 3 is set to benot smaller than 30 inches but not larger than 42 inches. If the lengthof the shaft 3 is too small, a turning radius of the swing becomessmall, and it becomes difficult to obtain a sufficient head speed. As aresult, the ball speed cannot be increased, and the flight distance ofthe ball cannot be extended. Therefore, the length of the shaft 3 ispreferably not smaller than 32 inches, and further preferably notsmaller than 34 inches. On the other hand, if the length of the shaft 3is too large, the inertia moment at the grip end becomes large, and apowerless golfer can become easily overwhelmed in terms of power.Therefore, the head speed cannot be increased, and the flight distanceof the ball cannot be extended. Thus, the length of the shaft 3 ispreferably not larger than 41.5 inches, and further preferably notlarger than 41 inches.

Furthermore, although the position of the center of gravity itself ofthe shaft 3 is not particularly limited in the present invention, it isordinarily within a range of, for example, for a shaft whose length is39 inches, 595 to 505 mm from the tip end 3 a (front end) of the shaft3. If the position of the center of gravity G of the shaft 3 is locatedcloser than 505 mm from the front end of the shaft 3, there is a highpossibility of not being able to increase head speed since the ease ofswinging the club is not improved due to the position of the center ofgravity not being sufficiently moved in the hand side direction.Therefore, the position of the center of gravity of the shaft 3 from thefront end of the shaft 3 is preferably not closer than 510 mm, andfurther preferably not closer than 515 mm. On the other hand, if theposition of the center of gravity G of the shaft 3 is farther than 595mm from the front end of the shaft 3, there is a high possibility ofstrengths such as flexural strength being insufficient due to a smallthickness on the front end side of the shaft. Therefore, the position ofthe center of gravity of the shaft 3 from the front end of the shaft 3is preferably not farther than 585 mm, and further preferably notfarther than 575 mm.

In the present invention, when the distance from the front end of theshaft 3 to the center of gravity G of the shaft is L_(G) and when thefull length of the shaft 3 is L_(S), a center-of-gravity rate Y (%)obtained as L_(G)/L_(S) satisfies 51.0≦Y≦60.0.

If the center-of-gravity rate Y is lower than 51.0(%), since the centerof gravity of the shaft is located close to the front end side of theshaft, the weight of the head has to be reduced in order to obtain aswing balance equivalent to that obtained from a hitherto known club,and the degree of freedom in designing a head becomes small. Thus, theinertia moment of the head becomes small, and a technique for loweringthe center of gravity cannot be implemented. Therefore, it becomesdifficult to achieve a large ball flight distance. Hence, thecenter-of-gravity rate Y is preferably not lower than 52.0, and furtherpreferably not lower than 53.0.

On the other hand, if the center-of-gravity rate Y is higher than 60.0,since the center of gravity of the shaft is located too close to theback end side of the shaft, even if the weight of the head isexcessively increased in order to obtain a level of ease of swingingequivalent to that obtained from a hitherto known club, i.e., the samelevel of swing balance; it is still difficult to perform a swing sincethe club weight is also increased. In addition, since the weight isallocated on the back end side rather than on the front end of theshaft, the strength of the front end side of the shaft becomes small.Therefore, the center-of-gravity rate Y is preferably not higher than58.0, and further preferably not higher than 56.0.

Furthermore, in the present embodiment, a thickness t of a portion up to150 mm from the butt end 3 b of the shaft 3 toward the front end side ofthe shaft is set to be not smaller than 0.30 mm but not larger than 0.40mm.

If the thickness t is smaller than 0.3 mm, since the center of gravityof the shaft is located close to the front end side of the shaft, theweight of the head has to be reduced in order to obtain a swing balanceequivalent to that obtained from a hitherto known club, and the degreeof freedom in designing a head becomes small. Thus, the inertia momentof the head becomes small, and a technique for lowering the center ofgravity cannot be implemented. Therefore, it becomes difficult toachieve a large ball flight distance. Hence, the thickness t ispreferably not smaller than 0.32 mm, and further preferably not smallerthan 0.34 mm.

On the other hand, when the thickness t is larger than 0.4 mm, since thecenter of gravity of the shaft is located too close to the back end sideof the shaft, even if the weight of the head is excessively increased inorder to obtain a level of ease of swinging equivalent to that obtainedfrom a hitherto known club, i.e., the same level of swing balance; it isstill difficult to perform a swing since the club weight is alsoincreased. In addition, since the weight is allocated on the back endside rather than on the front end of the shaft, the strength of thefront end side of the shaft becomes small. Therefore, the thickness t ispreferably not larger than 0.38 mm, and further preferably not largerthan 0.36 mm.

Furthermore, in the present embodiment, an outer diameter d of a portionup to 150 mm from the butt end 3 b of the shaft 3 toward the front endside of the shaft is set to be not smaller than 14.5 mm but not largerthan 15.3 mm.

If the outer diameter d is smaller than 14.5 mm, a flexural rigidityvalue EI of the butt side of the shaft becomes too low and the shaftbecomes soft. In addition, since directional stability of a hit balldeteriorates, the feel obtained when hitting a ball deteriorates.Therefore, the outer diameter d is preferably not smaller than 14.7 mm,and further preferably not smaller than 14.9 mm.

On the other hand, if the outer diameter d is larger than 15.5 mm, theflexural rigidity value EI of the butt side of the shaft becomes toohigh, and the feel obtained when hitting a ball deteriorates since theshaft bends little and is felt as being hard. Therefore, the outerdiameter d is preferably not larger than 15.4 mm, and further preferablynot larger than 15.3 mm.

Furthermore, in the present embodiment, the number of steps included ina portion up to 400 mm from the butt end 3 b of the shaft 3 toward thefront end side of the shaft is set to be not less than 5 but not morethan 8.

If the number of the steps is less than 5, the flexural rigidity valueEI of the butt side of the shaft becomes too high, and the feel obtainedwhen hitting a ball deteriorates since the shaft bends little and isfelt as being hard. Therefore, the number of the steps is preferably notless than 6.

On the other hand, if the number of the steps is more than 8, theflexural rigidity value EI of the butt side of the shaft becomes too lowand the shaft becomes soft. In addition, since directional stability ofa hit ball deteriorates, the feel obtained when hitting a balldeteriorates. Therefore, the number of the steps is preferably not morethan 7.

In addition, in the present embodiment, when compared to a portion up to600 mm from the tip end 3 a of the shaft toward the butt end 3 b side,the contained amount of retained austenite in the material of the shaftis set to be larger in a portion toward the butt end 3 b from this600-mm part. That is, the contained amount of retained austenite isdifferent in the tip end side of the shaft and in the butt end side ofthe shaft, with the boundary therebetween being the part 600 mm from thetip end 3 a of the shaft toward the butt end 3 b side. Morespecifically, the contained amount of retained austenite in the portionon the butt end side of the 600-mm boundary is set to be larger than thetip end side of the boundary.

If the “boundary” is located closer than 600 mm from the tip end 3 a ofthe shaft with respect to the butt end 3 b side, the flexural rigidityvalue EI of the butt side of the shaft becomes too low and the shaftbecomes soft. In addition, since directional stability of a hit balldeteriorates, the feel obtained when hitting a ball deteriorates.Therefore, the “boundary” is located preferably not closer than 650 mm,and further preferably not closer than 700 mm.

On the other hand, if the “boundary” is located farther than 900 mm fromthe tip end 3 a of the shaft toward the butt end 3 b, the flexuralrigidity value EI of the butt side of the shaft becomes too high, andthe feel obtained when hitting a ball deteriorates since the shaft bendslittle and is felt as being hard. Therefore, the “boundary” is locatedpreferably not farther than 850 mm, and further preferably not fartherthan 800 mm.

Example

Next, the steel shaft of the present invention will be described basedon Examples; however, the present invention is not limited only to thoseExamples.

15 golfers having an average head speed of 42 m/s were each asked toactually hit ten balls by using a 5-iron (club length: 38 inches, clubweight: 400 g) having a shaft with a specification shown in Table 1, andwere asked to answer a question regarding the feel obtained when hittinga ball. The golfers were asked to take into consideration ease ofswinging, directivity of a hit ball, and flight distance to perform a5-scale evaluation. An evaluation obtained from the most number ofgolfers was adopted.

5 points: Very good

4 points: Good

3 points: Average

2 points: Bad

1 point: Very bad

A shaft front end strength (T-point strength) was measured in accordancewith a testing method defined by SG mark. SG-type three point flexuralstrength is a SG-type breaking strength determined by the ConsumerProduct Safety Association. FIG. 2 is an illustrative diagram for themethod for measuring the SG-type three point flexural strength. As shownin FIG. 2, load F was applied downward from above at a load point t3while the shaft 3 was supported from below at two support points t1 andt2. The position of the load point t3 was a position dividing, into twoequal parts, the interval between the support point t1 and the supportpoint t2. The load point t3 was matched with the point (T-point) thatwas to be measured, and measurement was conducted.

Results of evaluation of feel of hitting a ball, flexural strength of ashaft front end portion, and flight distance are shown in Tables 1 to 3.

TABLE 1 Comparative Comparative Example 1 Example 2 Example 1 Example 3Example 2 Shaft weight [g] 95 95 95 95 95 Shaft full length [inch] 38 3838 38 38 Center-of-gravity rate [%] 49 52 55 58 61 Thickness [mm] of aportion 0.28 0.32 0.35 0.38 0.41 from butt end to 150 mm therefrom Outerdiameter [mm] of a portion 15 15 15 15 15 from butt end to 150 mmtherefrom Number of steps 5 5 5 5 5 Contained amount of retainedaustenite Uniform Uniform Uniform Uniform Uniform Feel evaluation 1 2 34 5 Flexural strength (T-point) [kgf] 240 230 220 190 160 of front endpart Flight distance [yard] 162 167 170 168 160

TABLE 2 Example 4 Example 5 Example 1 Example 6 Example 7 Shaft weight[g] 95 95 95 95 95 Shaft full length [inch] 38 38 38 38 38Center-of-gravity rate [%] 55 55 55 55 55 Thickness [mm] of a portion0.35 0.35 0.35 0.35 0.35 from butt end to 150 mm therefrom Outerdiameter [mm] of a portion 14.4 14.7 15 15.3 15.6 from butt end to 150mm therefrom Number of steps 5 5 5 5 5 Contained amount of retainedaustenite Uniform Uniform Uniform Uniform Uniform Feel evaluation 5 4 32 1 Flexural strength (T-point) [kgf] 220 220 220 220 220 of front endpart Flight distance [yard] 162 165 170 165 162

TABLE 3 Example 8 Example 1 Example 9 Example 10 Example 11 Shaft weight[g] 95 95 95 95 95 Shaft full length [inch] 38 38 38 38 38Center-of-gravity rate [%] 55 55 55 55 55 Thickness [mm] of a portion0.35 0.35 0.35 0.35 0.35 from butt end to 150 mm therefrom Outerdiameter [mm] of a portion 15 15 15 15 15 from butt end to 150 mmtherefrom Number of steps 4 5 6 5 5 Contained amount of retainedaustenite Uniform Uniform Uniform Larger in Larger in tip end butt endside side Feel evaluation 2 3 4 2 4 Flexural strength (T-point) [kgf]220 220 220 210 220 of front end part Flight distance [yard] 162 170 168168 168

REFERENCE SIGNS LISTS

-   -   1 iron type golf club    -   2 head    -   3 shaft    -   3 a tip end    -   3 b butt end    -   4 grip    -   5 shaft hole    -   6 hosel    -   7 grip hole    -   G center of gravity of a shaft    -   L_(G) distance from a tip end of a shaft to the center of        gravity of the shaft    -   L_(S) shaft full length

What is claimed is:
 1. A golf club steel shaft in which, when a distancefrom a front end of the shaft to a center of gravity point of the shaftis L_(G) and when a full length of the shaft is L_(S), acenter-of-gravity rate Y (%) obtained as L_(G)/L_(S) satisfies51.0≦Y≦60.0, a shaft weight X (g) satisfies 80≦X≦140, and a full lengthof the shaft L_(S) (inch) satisfies 30≦L_(S)≦42.
 2. The golf club steelshaft according to claim 1, wherein a thickness of a portion up to 150mm from a butt end toward a front end side of the shaft is not smallerthan 0.30 mm but not larger than 0.40 mm.
 3. The golf club steel shaftaccording to claim 1, wherein an outer diameter of a portion up to 150mm from a butt end toward a front end side of the shaft is not smallerthan 14.5 mm but not larger than 15.3 mm.
 4. The golf club steel shaftaccording to claim 2, wherein an outer diameter of a portion up to 150mm from a butt end toward a front end side of the shaft is not smallerthan 14.5 mm but not larger than 15.3 mm.
 5. The golf club steel shaftaccording to claim 1, wherein the number of steps included in a portionup to 400 mm from a butt end toward a front end side of the shaft is notless than 5 but not more than
 8. 6. The golf club steel shaft accordingto claim 2, wherein the number of steps included in a portion up to 400mm from a butt end toward a front end side of the shaft is not less than5 but not more than
 8. 7. The golf club steel shaft according to claim3, wherein the number of steps included in a portion up to 400 mm from abutt end toward a front end side of the shaft is not less than 5 but notmore than
 8. 8. The golf club steel shaft according to claim 4, whereinthe number of steps included in a portion up to 400 mm from a butt endtoward a front end side of the shaft is not less than 5 but not morethan 8.